1. Field of the Invention
The present invention relates to a transfer robot. In particular, the present invention relates to a transfer robot for transporting a thin platy work along a straight path.
2. Description of Related Art
Transfer robots are used, for example, in manufacturing processes of semiconductor devices and liquid crystal display devices, for automated transportation of thin, plate-like works (wafers, glass substrates and so on) into and out of predetermined process chambers. Very often, such transfer robots have a mechanism of moving the work along a straight path (linear moving mechanism). Generally, such a linear transfer robot is simpler in construction than so called multi-joint robots, and therefore cheaper. Japanese Patent Laid-Open No. 10-6258 discloses an example of the linear moving mechanism used in the transfer robot.
As shown in FIG. 36 of the present application, the linear moving mechanism includes two parallelogram links for example, which work in combination. Specifically, on a base plate 91, a first main link arm 92 is pivotably supported around a shaft O1, and a first assisting link arm 93 is pivotably supported around a shaft O2. The first main link arm 92 and the first assisting link arm 93 have tips connected to a middle plate 96. The first main link arm 92 is pivotable around a shaft O3. The first assisting link arm 93 is pivotable around a shaft O4. The base plate 91, the first main link arm 92 the first assisting link arm 93 and the middle plate 96 constitute a first parallelogram link. Likewise, a second parallelogram link is constituted by the middle plate 96, a second main link arm 94, a second assisting link arm 95 and a work supporting base 97. Specifically, the second main link arm 94 is connected to pivot around the shaft O3, and the second assisting link arm 95 is connected to pivot around a shaft O5, to the middle plate 96. Likewise, the second main link arm 94 is connected to pivot around a shaft O6, and the second assisting link arm 95 is connected to pivot around a shaft O7, to the work supporting base 97. The second main link arm 94 has a length equal to that of the first main link arm 92. The work supporting base 97 is provided with a plurality of prongs for carrying a thin, plate-like work.
Pivotal movement of the first main link arm 92 around the shaft O1 and pivotal movement of the second main link arm 94 around the shaft O3 are interlinked by an unillustrated transmission mechanism. Specifically, as the first main link arm 92 pivots in a predetermined direction (e.g. clockwise direction) around the shaft O1, the second main link arm 94 pivots in the opposite direction (counterclockwise) around the shaft O3. Further, the second main link arm 94 pivots at an angular speed twice an angular speed of the first main link arm 92. Further, as described above, the first main link arm 92 and the second main link arm 94 have the same length. With such a construction, even if the two parallelogram links are deformed (while the work is being transported) the work supporting base 97 maintains a constant orientation and therefore so does the prongs 98.
Recently, for example, the wafers used in manufacture of semiconductors are larger in outside diameter than before. Similarly, in manufacture of liquid crystal displays, there is an increase in panel sizes. These require a longer distance for the works to be transported, which poses a problematic situation. Namely, in the linear moving mechanism as shown in FIG. 36, weight of the work and of the prongs which carry the work causes a tendency that the entire link arm bends in the vertical direction. (The tendency becomes apparent especially when the link arm is extended.) As a result, it becomes difficult to perform the liner transportation of the work appropriately.
In order to solve this problem without changing the basic structure of the transfer robot, it is unavoidable that more rigid materials be used in many parts of the transfer robot and/or more high-precision parts be used e.g. in the bearing. However, these will lead to a significant cost increase.
The construction in FIG. 36 also has the following problem: Specifically, if a heated work W is placed on the prongs 98, radiant heat from the work W causes thermal expansion in the second main link arm 94 and the first main link arm 92. Since the second main link arm 94 is closer to the work W than is the first main link arm 92, thermal expansion in the second main link arm 94 is greater than in the first main link arm 92. As a result, the second main link arm 94 and the first main link arm 92 become unequal to each other in the length, causing a problem that the work W cannot be moved smoothly along the straight moving path. There is another problem: Specifically the length of the moving path is dependent on the length of the first main link arm 92 and the length of the second main link arm 94. However, according to the construction in FIG. 36 (where the length of the first main link arm 92 and the length of the second main link arm 94 are the same), both of the first main link arm 92 and the second main link arm 94 must be made longer if the transportation distance of the work W is to be increased.